Neural probes and interfaces are an essential tool in neuroscience. They typically comprise a multi-electrode array (MEA) configuration with exposed metal pads or electrodes located on rigid silicon shanks and connected, via interconnection traces, to output leads or to signal processing circuitry on a monolithic substrate. The exposed metal pads/electrodes provide a direct electrical interface with the neurons of a biological entity's nervous system to stimulate and/or record neural activity. Such neural probes enable researchers and clinicians to better explore and understand neurological diseases, neural coding, neural modulations, and neural topologies, as well as treat debilitating conditions of the nervous system. Moreover, implantable neural probes and interfaces in particular enable extended interaction with neural tissue. However, such probes and devices typically require invasive surgeries for implantation, and also often require additional surgeries to remove and replace devices that have failed or otherwise require maintenance or servicing. The medical industry continues to search for methods to improve patient comfort and device reliability, including developing methods to reduce the size and extend the lifetime of chronically implanted devices.
Standard polymer-based MEAs are made using multiple layers of polymers coated layer by layer (typically by spin-coating) after each metal film deposition and patterning, to insulate the patterned conductive wiring and lines. However, the multiple polymer-polymer and polymer-metal interfaces provide opportunities for water and solution infiltration leading to delamination and separation of the polymer layers. Penetration of bodily fluids at these compromised areas can result in trace metal corrosion, electrical shorts between interconnects, and ultimately device failure. Therefore, maintaining adequate adhesion between adjacent layers of polymer and polymer-metal layers is a consistent concern using this approach, and one of the reasons thin-film microelectrode arrays have been unsuccessful for long term implantation.
Previous attempts to solve the delamination problem for such polymer-based MEAs have included various polymer treatment methods to alter the physical or chemical properties of the polymer layers to improve adhesion therebetween. However, these methods have largely failed by either not providing sufficient improvements in adhesion, or the polymer treatment parameters required for effective adhesion is detrimental to the metal thin films. What is needed therefore is a durably insulated polymer-based MEA, and a method of fabricating such MEAs using reproducible microfabrication processes which extends operational lifetime by reducing the number and area of interfaces of the MEA that can be infiltrated and thereby reducing the opportunities for failure.